Polycarbonate resins have high impact resistance with ductility to notch or crack propagation at an average of up to about 0.2 inches thickness when the incident notch is 10 mils (thousandths of an inch) in radius. Above this average thickness the impact resistance and ductility of polycarbonate resins decline. This phenomena is commonly found in glassy plastics and is referred to as the critical thickness for notched impact resistance of a glassy plastic.
In addition, the impact strength of notched polycarbonate resins decreases as temperatures decrease below about -5.degree. C. and also after aging the polymers at elevated temperatures above about 100.degree. C. These temperatures are commonly found in applications where extremes of heat and cold are to be expected.
Thus, it is desirable to modify a polycarbonate resin composition to extend the impact strength and ductility of polycarbonate resins to variable thickness parts or articles of use which resist embrittlement upon exposure to high or low temperatures in a notched or scratched condition.
Compositions are known which extend the high impact resistance and ductility characteristics of polycarbonate resins to parts beyond the critical thickness and under low and high temperature aging conditions, but many of these compositions suffer from incompatibilities of the polymeric components which results in poor weldline or knit line strength in fabricated parts as evidenced by low double-gate impact strengths when measured according to ASTM D256.
The applicant has found that the use of a minor amount of a hydrogenated AB diblock copolymer, wherein one block is derived from isoprene and the other from an alkenyl aromatic compound, e.g., styrene, in combination with a polycarbonate resin, will improve the impact strength of the polycarbonate resin. These compositions are compatible and have good surface characteristics. They may also include a minor amount of a multiphase composite interpolymer which comprises a C.sub.1-5 acrylate and a C.sub.1-5 methacrylate.